1. Field of the Invention
The present invention relates to an optical device that monolithically integrates an optically active device and an optically passive device.
2. Related Prior Art
Japanese patent published as S63-196088 has disclosed a semiconductor laser diode, an edge portion of which is widened in the band-gap energy by the diffusion of zinc (Zn) atoms. This edge portion functions as a window region for the coherent light. This window region with widened band-gap energy may prevent the laser from the COD (Catastrophic Optical Damage) and the degradation thereby.
Japanese patent published as 2001-148531 has disclosed an optically integrated device that includes an optical waveguide and an optical amplifier both provided on single GaAs substrate. The optical amplifier, optically coupled with the optical waveguide, comprises of an active layer made of GaxIn1-xNyAs1-y, first and second cladding layers sandwiching the active layer therebetween. The waveguide comprises a core made of GaInNAs or GaAs, and first and second cladding layers sandwiching the core.
Optical integrated devices applicable in a wavelength range longer than 1 μm may be processed on currently available InP substrate with 3-inch diameter. A semiconductor materials with greater band-gap energy than that of Inp does not lattice-match to InP. Accordingly, in the InP system, which means that semiconductor materials considered have a lattice constant matching to that of InP, materials having comparably greater band-gap energy may not apply to the optical confinement layer and the cladding layer. This means that the band-gap difference between the active layer and layers surrounding the active layer, such as cladding layer and optical confinement layer, is not ensured, thereby reducing the carrier confinement into the active layer and degrading the performance of the device against the temperature.
The former Japanese patent, S63-196088, has related to the facet protection of the optical cavity, not relates to the optical integrated device. While, the latter Japanese patent, 2001-148531, has related to the optical integrated device applicable in the longer wavelength band. This integrated device includes an active layer made of GaInNAs related material. Since the lattice constant of GaInNAs matches to that of the GaAs, the GaAs wafer with relatively large size, 6-inches, may be used. Moreover, since the GaAs related material, such as AlGaAs and AlGaInP and having comparably greater band-gap energy than InP, may be applicable to the cladding layer and the confinement layer, the band-gap difference between the active layer and the cladding layer becomes large, accordingly, the carrier confinement within the active layer becomes effective. Therefore, in the optical integrated device using GaInNAs, the performance against the temperature may be drastically improved as compared with the InGaAs(P)/InP system.
In such optical device integrating the active device with the passive device, the light processed in the active device is necessary to be not absorbed in the passive device. Accordingly, the band-gap energy of the passive device must be greater than that of the active device. On the other hand, both devices must be smoothly coupled to each other to eliminate the reflection of light at the interface therebetween. Therefore, the latter Japanese patent has disclosed a butt joint structure, in which the optically active layers of both devices are physically come into contact after independently processed or one of active layers is processed to come into contact to the other active layer that is processed in advance.
However, in this butt joint structure, the layer structure in the active device and that in the passive device are occasionally different to each other, at least two structures may not be completely identical in the physical dimensions to each other, so the mode field diameter of the light in the active device and that in the passive device becomes different, accordingly, the reflection of light is inevitably induced at the interface.
Moreover, in the butt joint structure, the passive device is independently formed after the formation of the active device, i.e. the epitaxial layers for the passive device is, after the etching of layers for the active device, grown on thus etched portion. However, an extraordinary layer may be formed at the second growth, and this extraordinary layer degrades the interface and the optical coupling thereof which increase the reflection at the interface. Thus, the butt joint structure lacks the reliability and degrades the performance of the optical integrated device.